The present invention relates to the field of rock bits and inserts therefor. More particularly, the invention relates to the field of roller cone type bits and percussion type bits which include inserts with a layer of polycrystalline diamond material on an insert body.
Roller cone rock bits are widely used for oil, gas, and geothermal drilling operations. In general, roller cone rock bits include a body connected to a drill string and typically three hollow cutter cones each mounted on journals on the bit body for rotation about an axis transverse to the axis of the drill bit. In use, the drill string and bit body are rotated in the bore hole and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled.
Roller cone rock bits are generally divided into two categories: those used with mud as the drilling fluid, and those used with air as the drilling fluid. Although similar in the basic design, these two types of roller cone rock bits also have many design and manufacturing dissimilarities due to the differences in how the bits are used as well as the kinds of drilling equipment that is used with these two types of bits.
Typically, mud is used as the drilling fluid when drilling in formations that would tend to close in on the hole that has been drilled. That is, the weight of the mud is used to maintain the integrity of the borehole by balancing the geophysical forces surrounding the bore hole. As used herein, the term "mud" is intended to have a relatively broad meaning including conventional drilling mud, water, brine, and oils, as well as mixtures thereof.
On the other hand, air is typically used when drilling in fractured formations where the mud would have a tendency to seep into the formation, and when the borehole integrity is sufficiently stable.
Because typical drilling mud is relatively abrasive, roller cone rock bits used with mud generally include an elastomer seal to protect the bearings from the drilling mud. Also, mud bits are generally designed to last much longer and typically include precision journal bearings and a lubricant reservoir with pressure compensation means.
In contrast, air bits are generally designed for shorter run times and include unsealed unlubricated roller bearings. Accordingly, air bits are often used for geothermal drilling because the high temperatures encountered in this type of drilling would usually degrade the elastomer seals and lubricants used in the design of mud bits.
In addition, because the weight of the column of drilling mud above applies a greater pressure than a column of air on the bottom of the borehole, the interaction between the cutting inserts and the bottom of the hole is different for the inserts in a roller cone mud bit and the inserts in a roller cone air bit. In particular, the inserts of a roller cone mud bit are typically subjected to higher dynamic forces due to the influence of the mud column on the borehole bottom. Also, because the mud acts to balance the geophysical pressures surrounding the borehole, including the hole bottom, mud drilling typically has a slower rate of penetration than air drilling. Consequently, under identical weight on bit and rotational speed, inserts on mud bits will typically contact the rock formation more times to drill a given equivalent footage than in air drilling. Also, the inserts in mud bits typically extend further from the cone to achieve a more aggressive cutting action than is typically found with air bits.
In contrast, because the bottom of the borehole is underpressure balanced when drilling with air, the rock tends to explode on contact with the inserts. As a consequence of the explosive nature of air drilling, the peak load on each insert is lower than that with mud drilling.
Fixed head percussion rock bits, some times known as hammer bits, are another type of rock boring tool. Percussion rock bits are used most often in drilling blast holes for mining and construction. Other uses for fixed head percussion bits include gas, oil, and water drilling. The percussion bits include a body with one end for connecting to an air hammer. Hard metal inserts are embedded in the other end.
In operation, the air hammer moves the bit up and down rapidly. The percussion bit hammers the inserts against the rock being drilled, shattering it by repeated blows. A typical air hammer for percussion bits operates at about 2,000 blows per minute while being rotated at about 60 r.p.m. Compressed air pumped through the bit removes chips of fractured rock from the hole being drilled. Some percussion bits are driven by hydraulic action.
A significant improvement in the life expectancy of roller cone and percussion rock bits involves the use of cemented metal carbide inserts put into the roller cones for crushing rock on the bottom of the bore hole. Naturally, cemented metal carbide, such as cobalt cemented tungsten carbide, offered improved wear resistance over steel along with sufficient toughness to withstand the forces encountered during drilling. Since the advent of cemented metal carbide inserts in rock drilling, much effort has been devoted to improving both the wear resistance and toughness of the inserts. Wear resistance is important to prevent the insert from simply wearing away during drilling. Toughness is important to avoid inserts breaking off due to the high impact loads experienced in drilling.
A more recent development in roller cone rock bit inserts has been the use of a layer of polycrystalline diamond (PCD). In particular, inserts have been fabricated which include an insert body made of cobalt bonded tungsten carbide and a layer of polycrystalline diamond directly bonded to the protruding head portion of the insert body. The term polycrystalline diamond generally refers to the material produced by subjecting individual diamond crystals to sufficiently high pressure and high temperature that intercrystalline bonding occurs between adjacent diamond crystals. Naturally, PCD offers the advantage of greater wear resistance. However, because PCD is relatively brittle, some problems have been encountered due to chipping or cracking in the PCD layer.
U.S. Pat. No. 4,694,918, assigned to the assignee of the present invention, discloses roller cone rock bits and inserts therefor which inserts include a cemented metal carbide insert body, an outer layer of polycrystalline diamond, and at least one transition layer of a composite material. The composite material includes polycrystalline diamond and precemented metal carbide pieces. This transition layer between the outer layer of PCD and the head portion has been found to extend the life expectancy of PCD rock bit inserts by reducing the incidence of cracking an chipping.